Atomic force microscopy (AFM) allows high resolution surface imaging by scanning a sharp tip over a surface while controlling the interaction force between the tip and surface. AFM typically relies on a micromachined cantilever with a sharp tip at the distal or free end of the cantilever as a force sensor to detect the interaction between the tip and the sample. The force of the interaction of the tip is sensed by the resultant deflection of the cantilever on which the tip is mounted. The role of the cantilever is to translate the force acting on the tip into a deflection that can be subsequently monitored by various means. Several methods of detecting the deflection of the cantilever are available such as electron tunneling, capacitance, optical interferometry, optical polarization and optical deflection. Such cantilever probes can be used in other types of scanning microscopy.
Various methods for fabrication of a cantilever stylus with the integrated tip have been proposed since the invention of the AFM by Gerd Binnig, Calvin Quate and Christopher Gerber in 1986. Albrecht et al. in 1990 proposed several methods to fabricate thin film cantilevers with integrated pyramidal tips. Since then, many processes involving dry and wet etching have been proposed and developed for the fabrication of a cantilever-tip assembly. Of these, a prominent method described by Akamine et al. in U.S. Pat. No. 5,021,364 fabricates the cantilever stylus made of thin films with an integrated silicon tetrahedral tip. This method was intended for batch fabrication of the cantilever-tip assemblies for AFM applications. Toda in U.S. Pat. No. 5,386,110 proposes another method to overcome some process limitations of the previous method.
Akamine et al. propose fabricating a silicon membrane followed by lithographic definition of the cantilever on a membrane. To create a self-aligned perpendicularly extending sharp tetrahedral tip on the distal end of the cantilever, reactive ion etching (RIE) of the cantilever followed by thermal oxidation will lead to oxide growth only on the sidewalls, the top surface being protected by silicon nitride. The nitride is then selectively removed from the cantilever and anisotropically thin the silicon on the cantilever. The anisotropic etchant does not etch through the tetrahedral volume of silicon at the end of the cantilever since it is bound by the silicon nitride from the bottom, oxide on the sidewalls, and a (111) silicon crystal plane.
Although the Akamine process is very robust, it presents some inherent process difficulties. The thickness of membrane determines the thickness of the cantilever and the height of the tip. The definition of the membrane of exact thickness becomes very difficult and requires high level of control during wet etching. Lithography on a thin silicon membrane is very difficult, and the chances of wafer breakage are also very high during the lithography process. The method also requires precise alignment for selectively removing nitride from the cantilever to form the tip by anisotropic etching, which again is a drawback in batch fabrication of the cantilever chip with precision. Another limitation is the inherent inability of this process to fabricate rectangular single crystal silicon cantilevers, thus limiting its use to fabricate thin film cantilevers for a restricted set of applications.
As an alternative to the Akamine process, Toda proposes a method using a wafer having a etch stop layer as the starting wafer. In this process, silicon nitride films are deposited on the two opposed surfaces of the starting wafer. The silicon nitride film on the lower surface is selectively removed and silicon is anisotropically etched until the etch stop layer is reached. The exposed etch stop layer is then removed by wet or dry etching and silicon nitride film is deposited on the etched side. A rectangular pattern is defined on the front surface using photolithography. The silicon nitride film and the silicon inside the rectangular pattern is etched through the hole until the lower portion is reached. The exposed silicon sidewalls of the hole are then oxidized to form a silicon dioxide film. The silicon nitride film on the top surface is removed and the exposed silicon is etched using wet anisotropic etching, to obtain the tetrahedral tips on the distal end of the cantilever.
Though Toda's method addresses the problem evident in Akamine et al. of controlling the thickness of the membrane by using an etch stop layer, it still requires a lithography to be performed on a thin silicon layer to etch the hole. Hence, handling of the wafer becomes very critical during the lithography step. Also the method does not address the inability of previous processes of fabricating rectangular cantilevers made of single crystal silicon. Though Toda proposes an embodiment to fabricate silicon cantilevers using heavily doped boron layer as the etch stop layer and later using this layer for the cantilever, the implementation of such a scheme is expensive and complex, which increases the cost of mass fabrication.